1. Field of the Invention
This invention relates to an apparatus for connecting double vacuum pipes for conducting cryogenic fluids such as liquid helium or liquid nitrogen.
2. Description of the Related Art
Recently, the demand for liquid helium, which has a very low boiling point of -269.degree. C. has increased in the field of superconducting techniques. Double pipes having a large adiabatic effect or a large heat-insulating effect are used as pipes for supplying cryogenic fluid such as liquid helium from a large tank to a small tank. If an ordinary quick pipe coupler for connecting single pipes is used, the connecting portions of the coupler are cooled to a very low temperature and frost attaches thereto as the cryogenic fluid comes to contact with the inner peripheral surfaces of the coupler during the connecting process. Thus, this ordinary pipe coupler cannot hermetically connect the double vacuum pipes together. In overcoming this difficulty, a connecting apparatus employing a flange coupler and defrost means is usually been used, as disclosed in Japanese Patent Examined Application Publication 1-41879, Japanese Patent Unexamined Application Publication 56-63178 and Japanese Utility Model Unexamined Application Publication 60-85690.
The connecting apparatus employing a flange coupler can attain a satisfactory sealing effect but has the disadvantage that it takes too long to connect pipes. Further, if no defrost means is used, the attachment of frost to the coupler cannot be avoided. Moisture in the air is condensed by the cryogenic fluid and attaches as frost to the flange coupler of the connecting apparatus, making it difficult to connect the pipes. The frost on the flange portion is removed by blowing hot wind onto the flange portion or by gasifying the cryogenic fluid at room temperature until it reaches the flange coupler. The latter method, as disclosed in Japanese Patent Examined Application Publication 1-41879, is performed by a connector which comprises longer double vacuum pipes for evaporation connected end-to-end to each other and an outer pipe having a larger diameter than the double vacuum pipes and surrounding them. The cryogenic fluid is introduced in an annular space defined between the outer tube and the double vacuum pipes and become warm until it gasifies. Thus, the outer peripheral surface of the outer pipe is made contact with the outer atmosphere. Since this apparatus also contains a flange coupler, the double vacuum pipes can be neither rapidly connected to nor rapidly disengaged from each other.